Torque modulated transfer case

ABSTRACT

The present invention is directed to an improved &#34;on-demand&#34; power transfer system of the type having a transfer case incorporated into the driveline of a four-wheel drive vehicle. The transfer case is arranged to normally deliver drive torque to the driven wheels so as to establish a two-wheel drive mode of operation. In addition, the transfer mechanism is equipped with a clutch assembly for selectively delivering drive torque to the non-driven wheels for establishing an on-demand four-wheel drive mode of operation. The transfer mechanism is also equipped with a rotary actuator and a drive mechanism for actuating the clutch assembly. The power transfer system further includes sensors for detecting various dynamic and operational characteristics of the vehicle and generating sensor input signals indicative thereof. A controller is provided for processing the input signals and controlling actuation of the rotary actuator in response thereto. In one preferred construction, a shift mechanism is used in conjunction with the drive mechanism for selectively de-coupling the output shafts of the transfer case from its input shaft so as to establish a non-driven Neutral mode.

CROSS-REFERENCE

This is a continuation of U.S. Ser. No. 08/230,122, filed Apr. 19, 1994,now U.S. Pat. No. 5,400,866 which is a continuation-in-part of U.S. Ser.No. 08/028,952, filed Mar. 10, 1993, now U.S. Pat. No. 5,323,871.

BACKGROUND OF THE INVENTION

The present invention relates to a power transfer system for controllingthe distribution of drive torque between the front and rear wheels of afour-wheel drive vehicle as a function of various system andoperator-initiated inputs.

In view of increased consumer popularity in four-wheel drive vehicles, aplethora of power transfer systems are currently being utilized invehicular driveline applications for selectively directing power (i.e.,drive torque) to the non-driven wheels of the vehicle. In many powertransfer systems, a part-time transfer case is incorporated into thedriveline and is normally operable in a two-wheel drive mode fordelivering drive torque to the driven wheels. In addition, suchpart-time transfer cases also include a mechanical "mode" shiftmechanism which can be selectively actuated by the vehicle operator forrigidly coupling the non-driven wheels to the driven wheels forestablishing a part-time four-wheel drive mode. As will be appreciated,a motor vehicle equipped with a part-time transfer case offers thevehicle operator the option of selectively shifting between thetwo-wheel drive mode during normal road conditions and the part-timefour-wheel drive mode for operation under adverse road conditions.

Alternatively, it is known to use "on-demand" power transfer systems forautomatically directing power to the non-driven wheels, without anyinput or action on the part of the vehicle operator, when traction islost at the driven wheels. Modernly, it is known to incorporate the"on-demand" feature into a transfer case by replacing themechanically-actuated "mode" shift mechanism with a clutch assembly thatis interactively associated with an electronic control system and asensor arrangement. During normal road conditions, the clutch assemblyis maintained in a non-actuated condition such that drive torque is onlydelivered to the driven wheels. However, when the sensors detect a lowtraction condition at the driven wheels, the clutch assembly isautomatically actuated to deliver drive torque "on-demand" to thenon-driven wheels. Moreover, the amount of drive torque transferredthrough the clutch assembly to the non-driven wheels can be varied as afunction of specific vehicle dynamics, as detected by the sensorarrangement. One example of such an "on-demand" power transfer system isdisclosed in U.S. Pat. No. 4,773,500 to Naito, et al wherein ahydraulically-actuated clutch assembly is operable for automaticallycontrolling the amount of drive torque transferred to the non-drivenwheels as a function the wheel speed difference (i.e., the wheel slip)between the front and rear wheels. While numerous variations of suchhydraulically-actuated "on-demand" systems are known, they are ratherexpensive and complex in that they each require a dedicated source ofpressurized hydraulic fluid, electronically-controlled flow controlvalving and the associated hardware.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an improved powertransfer system of the type having a transfer mechanism incorporatedinto the driveline of a four-wheel drive vehicle. The transfer mechanismis arranged to deliver drive torque to the driven wheels so as toestablish a two-wheel drive mode of operation. In addition, the transfermechanism is equipped with a clutch assembly for selectively deliveringdrive torque to the non-driven wheels for establishing a four-wheeldrive mode of operation.

As a related object, the transfer mechanism is further equipped withrotary actuator means for actuating the clutch assembly. The rotaryactuator means is preferably mounted directly to the transfer mechanismto substantially minimize the packaging requirements associated with thepower transfer system while concomitantly enhancing in-servicereliability and allowing pre-assembly of the transfer mechanism prior tofinal installation into the vehicle driveline.

In a first preferred form, the power transfer system further includessensor means for detecting various dynamic and operationalcharacteristics of the vehicle and generating sensor input signalsindicative thereof, and controller means for controlling actuation ofthe rotary actuator means in response to the sensor input signals. Undermost normal road and tractive conditions, the clutch assembly ismaintained in a non-actuated condition such that drive torque is onlytransmitted to the driven wheels. However, upon the occurrence oftraction loss at the driven wheels, the clutch assembly is automaticallyactuated for transferring drive torque to the non-driven wheels, therebyestablishing "on-demand" four-wheel drive operation. In addition, theactuated condition of the clutch assembly is controllably modulated as afunction of the sensor input signals for automatically varying theamount of drive torque directed to the non-driven wheels. Thus, thepresent invention is directed to an "on-demand" power transfer systemcapable of providing instantaneous traction improvement and enhancedsteering control upon occurrence of unanticipated traction loss at thedriven wheels.

The power transfer system may further include means for establishing atwo-wheel drive mode and a part-time four-wheel drive mode in additionto an on-demand drive mode. To this end, mode select means are providedto permit a vehicle operator to select a desired one of the threeavailable drive modes and generate a mode signal indicative thereof. Themode signal is delivered to the controller means for controllingactuation of the rotary actuator means. When the two-wheel drive mode isselected, all drive torque is delivered to the driven wheels and theclutch assembly is maintained in the non-actuated condition. When thepart-time four-wheel drive mode is selected, the clutch assembly isfully actuated into a "lock-up" condition for distributing the drivetorque between the driven and non-driven wheels as dictated by thetractive forces generated at each respective set of wheels. When the"on-demand" mode is selected, the actuated condition of the clutchassembly is controllably modulated as a function of the sensor inputsignals for automatically varying the amount of drive torque directed tothe non-driven wheels. Thus, the power transfer system offers thevehicle operator the option of selecting the specific drive modebest-suited for operating the motor vehicle during normal or adverseroad conditions as well as for off-road recreational use.

According to yet another alternative embodiment of the power transfersystem, the transfer mechanism is equipped with means for establishing anon-driven or "Neutral" mode whereby no drive torque is transmitted tothe driven wheels. In association with this transfer mechanism, the modeselect means is adapted to permit the vehicle operator to select theNeutral mode in addition to one or more of the other available part-timeand/or on-demand drive modes. Thus, when the power transfer system isoperated in any drive mode other than the Neutral mode, a movable shiftsleeve is maintained in a first position for mechanically coupling inputand output members of the transfer mechanism for delivering drive torqueto the driven wheels. However, when the Neutral mode is selected, arepresentative mode signal is sent to the controller means forcontrolling actuation of the rotary actuator means so as to causemovement of the shift sleeve from the first position to a secondposition for disconnecting the input and output members of the transfermechanism. In the second position, no drive torque is transmittedthrough the transfer mechanism and thus, no power is supplied to thedriven wheels. In a further preferred form of the transfer mechanism,the clutch assembly is returned to its non-actuated condition when theNeutral mode is selected.

A further object of the present invention is to supply one or more"operator-initiated" input signals to the controller means for furthercontrolling "on-demand" operation of the power transfer system inresponse thereto. Preferably, the operator-initiated input signals areindicative of the position of a movable control element (i.e.,accelerator pedal, throttle position, steering wheel, brake pedal, etc.)and are used, in conjunction with the sensor input signals, foroptimizing the amount of drive torque delivered to the non-driven wheelsduring operation in the "on-demand" mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention willbecome apparent from analysis of the following written specification,the accompanying drawings and the appended claims in which:

FIG. 1 is a schematic representation of an exemplary four-wheel drivevehicle having the power transfer system of the present inventionincorporated therein;

FIG. 2 is a cross-sectional view of a transfer case constructedaccording to a first embodiment of the power transfer system and whichincludes a clutch assembly, a drive mechanism, and anelectronically-controlled rotary actuator;

FIG. 3 is an enlarged partial view of FIG. 2 showing the variouscomponents in greater detail;

FIG. 4 is a side view of a sector plate associated with the drivemechanism of FIGS. 2 and 3;

FIG. 5 is a block diagram of the control system for the power transfersystem of the present invention;

FIGS. 6 is a flow chart depicting a control sequence for the operationsperformed by the control system of FIG. 5;

FIG. 7 illustrates exemplary plots of relationships between wheel speeddifferential signals at various vehicle speed ranges and the electricalcontrol signal supplied by the control system to the rotary actuator forcontrolling the amount of torque transferred through the clutchassembly;

FIG. 8 is an exemplary plot of a relationship between steering angle anda control characteristic used for modifying the speed differentialsignal;

FIG. 9 graphically illustrates the relationship of the electricalcontrol signal with respect to the output force generated by the drivemechanism and the corresponding drive torque transferred through theclutch assembly to the non-driven wheels;

FIG. 10 is a flow chart, similar to the flow chart of FIG. 6, depictingthe control sequence for a power transfer system equipped with modeselection capabilities;

FIG. 11 is a sectional view of a transfer case constructed according toan alternative embodiment and having a modified drive mechanismincorporated therein;

FIG. 12 is a sectional view of a transfer case constructed in accordancewith yet another alternative embodiment;

FIG. 13 is an enlarged partial view of FIG. 12 showing the variouscomponents in greater detail;

FIG. 14 is a side view of a sector plate associated with the drivemechanism shown in FIGS. 12 and 13;

FIG. 15 is a partial sectional view of an alternative construction forthe transfer case shown in FIGS. 12 through 14 which incorporates amodified drive mechanism having means for establishing a "Neutral" mode;and

FIG. 16 is a side view of a sector plate associated with the modifieddrive mechanism shown in FIG. 15.

DETAILED DESCRIPTION OF THE INVENTION

In general, the present invention is directed to a power transfer systemwhich is operably installed between the driven and non-driven wheels ofa four-wheel drive vehicle. In operation, the amount of power (i.e.,drive torque) transferred to the non-driven wheels is controllablyregulated in accordance with various system and driver-initiated inputsfor optimizing the tractive characteristics of the vehicle whileconcomitantly enhancing overall steering control. In addition, the powertransfer system may also include means for permitting a vehicle operatorto select between a two-wheel drive mode, a part-time four-wheel drivemode, and an "on-demand" drive mode.

Referring to FIG. 1 of the drawings, a drivetrain for a four-wheel drivevehicle is schematically shown interactively associated with a powertransfer system 10 which incorporates the novel principles of thepresent invention. The motor vehicle drivetrain has a pair of frontwheels 12 and rear wheels 14 both drivable from a source of power, suchas an engine 16, through a transmission 18 which may be of either themanual or automatic type. In the particular embodiment shown, thedrivetrain is a rear wheel drive system which incorporates a transfercase 20 operable to receive drive torque from engine 16 and transmission18 for normally driving rear wheels 14 (i.e., the "driven" wheels) in atwo-wheel drive mode of operation. Front wheels 12 and rear wheels 14are shown connected at opposite ends of front and rear axle assemblies22 and 24, respectively. As is known, a rear differential 26 isinterconnected between rear axle assembly 24 and one end of a rear driveshaft 28, the opposite end of which is interconnected to a first outputmember 30 of transfer case 20. Similarly, front axle assembly 22includes a front differential 32 that is coupled to one end of a frontdrive shaft 34, the opposite end of which is coupled to a second outputmember 36 of transfer case 20. It is to be understood that the specificorientation of the drivetrain is merely exemplary in nature and that thedrivetrain could be reversed for normally driving front wheels 12.

According to each preferred embodiment of power transfer system 10,transfer case 20 is equipped with an electronically-controlled torquetransfer arrangement for delivering drive torque to front wheels 12(i.e., the non-driven wheels) for establishing a four-wheel drive modeof operation. More specifically, the torque transfer arrangementincludes a transfer clutch 38 that is operable for transferring drivetorque from first output member 30 to second output member 36, therebydelivering drive torque to front wheels 12. Power transfer system 10further comprises rotary actuator means 40 for actuating transfer clutch38, first sensor means 42 for sensing specific dynamic and operationalcharacteristics of the motor vehicle and generating sensor input signalsindicative thereof, and controller means 46 for generating a controlsignal in response to the sensor input signals. Moreover, controllermeans 46 is adapted to control the amount of drive torque transferredthrough transfer clutch 38 to second output member 36 by sending thecontrol signal to rotary actuator means 40. As is schematically shown,controller means 46 is also operable for illuminating a visual display48, located within the passenger compartment, for providing the vehicleoperator with a visual indication of the operational status of powertransfer system 10. As an additional feature, rotary actuator means 40may be provided with mode locking means for maintaining power transfersystem 10 in the selected drive mode upon the interruption of power.

Power transfer system 10 can include second sensor means 50 forgenerating "operator-initiated" input signals that are indicative of theposition of one or more movable control elements under the control ofthe vehicle operator. The operator-initiated input signals are used forestablishing control characteristics which, in conjunction with thesensor input signals, further regulate the torque distribution during"on-demand" operation. As a further option, power transfer system can beequipped with mode select means 44 for permitting the vehicle operatorto select one of a two-wheel drive mode, a part-time four-wheel drivemode and an "on-demand" drive mode. In a system equipped with modeselect means 44, rotary actuator means 40 is operable for actuatingtransfer clutch 38 in response to a mode signal generated by the vehicleoperator. When the two-wheel drive mode is selected, all drive torque isdelivered from first output member 30 to rear wheels 14 and transferclutch 38 is maintained in a "non-actuated" condition. When thepart-time four-wheel drive mode is selected, transfer clutch 38 is fullyactuated and maintained in a "lock-up" condition such that second outputmember 36 is, in effect, rigidly coupled for driven rotation with firstoutput member 30. As such, the torque distribution between front wheels12 and rear wheels 14 is dictated by the specific tractive forcesgenerated at each wheel set. When the "on-demand" drive mode isselected, controller means 46 controls the degree of actuation of rotaryactuator means 40 for varying the amount of drive torque directed tofront wheels 12 through transfer clutch 38 as a function of the sensorinput signals for providing enhanced traction when needed. In addition,the ability to controllably modulate the actuated condition of transferclutch 38 also provides for superior handling and steering control bysubstantially minimizing the oversteer and understeer tendencies of thevehicle during a cornering maneuver, such tendencies known to becommonly associated with two-wheel drive operation and part-timefour-wheel drive operation, respectively. Other advantages associatedwith controllably modulating the actuated state of transfer clutch 38will be detailed hereinafter.

With particular reference to FIGS. 2 and 3, a preferred construction fortransfer case 20 will now be described. Transfer case 20 is shown toinclude a housing 56 formed by a series of modular sections that aresuitably interconnected in a conventional manner. A transmission outputshaft 58 couples transmission 18 to an input shaft 60 of transfer case20 for supplying power thereto. Input shaft 60 has an annular input gear62 formed integral therewith. In the embodiment shown, first outputmember 30 is an elongated mainshaft 64 which is aligned on thelongitudinal axis of input shaft 60 and is supported for rotation withinhousing 56. An intermediate sleeve 66 is concentrically supported on aforward end portion of mainshaft 64 and is fixed for rotation therewithby means of one or more sets of sleeve internal splines 68 engaged withcorresponding sets of external splines 70 formed on mainshaft 64. Inaddition, intermediate sleeve 66 is formed with external splines 72 thatare shown to be engaged with internal splines 74 formed on input gear62. As such, drive torque is transferred without reduction from inputshaft 60 to mainshaft 64 via intermediate sleeve 66. While transfer case20 is shown to utilize a separate intermediate sleeve 66, it iscontemplated that sleeve 66 could be integrated into mainshaft 64 suchthat mainshaft 64 would be coupled directly to input shaft 60.

With continued reference to FIGS. 2 and 3, means are shown fortransferring drive torque from mainshaft 64 to front wheels 12 throughtransfer clutch 38. More specifically, a drive sprocket 76 is shownfixed (i.e., splined) for rotation on a tubular extension 78 of arotatable clutch hub 80 that is associated with transfer clutch 38.Extension 78 is shown rotatably supported on mainshaft 64 by one or moresuitable bearing assemblies 82. Drive sprocket 76 drivingly engages achain 84 which is coupled to a lower driven sprocket 86. Driven sprocket86 is coupled to, or an integral portion of, second output member 36 oftransfer case 20. Second output member 36 is shown as a front outputshaft 88 which is supported for rotation within housing 56 by suitablebearing assemblies 90 and 92. As noted, front output shaft 88 isoperably connected to the motor vehicle's front wheel 12 via front driveshaft 34.

Transfer clutch 38 is shown operably installed within transfer case 20for selectively transferring drive torque from intermediate sleeve 66 tofront output shaft 88. Preferably, transfer clutch 38 is amechanically-actuated multi-plate clutch assembly that is arranged toconcentrically surround a portion of mainshaft 64 and intermediatesleeve 66. As noted, clutch hub 80 is fixedly secured to drive sprocket76 so as to drive, or be driven by, front output shaft 88 of transfercase 20. In a preferred form, transfer clutch 38 also includes arotatable drum assembly 94 concentrically surrounding clutch hub 80 andis fixed (i.e., splined) to intermediate shaft 66 for rotationtherewith. Drum assembly 94 has an outer cylindrical drum 96 which isenclosed at one end by a cover plate 98. As seen, cover plate 98 has acup-shaped annular portion 100 surrounding the aft end of input gear 62and which forms an inward radial flange 102 having internal splines 104meshed with external splines 72 of intermediate sleeve 66. Accordingly,drum assembly 94 is coupled for rotation with intermediate sleeve 66 andmainshaft 64. Thus, drum assembly 94 and clutch hub 80 are capable ofrotating relative to one another and form an internal chamber 106therebetween. Disposed within internal chamber 106 are two sets ofalternately interleaved friction clutch plates that are operable fortransferring torque from input shaft 60 through intermediate sleeve 66and drum assembly 94 to clutch hub 80 and, ultimately, to front outputshaft 88 in response to a clutch "engagement" force applied thereto. Oneset of clutch plates, referred to as inner clutch plates 108, aremounted (i.e., splined) for rotation with clutch hub 80 while the secondset of clutch plates, referred to as outer clutch plates 110, aremounted (i.e. splined) to outer drum 96 for rotation with drum assembly94. In addition, a reaction plate 112 is mounted on the aft end of outerdrum 96 for enclosing the interleaved clutch plates within chamber 106.Reaction plate 112 is rotatable with drum assembly 94 and yet is axiallymovable with respect to interleaved friction clutch plates 108 and 110.Thus, reaction plate 112 acts as a pressure plate for compressing theinterleaved clutch plates so as to cause drive torque to be transferredthrough transfer clutch 38 as a function of the clutch engagement forceexerted thereon by rotary actuator means 40. In the embodiment shown,reaction plate 112 is supported for limited axial movement around theouter peripheral surface of an intermediate portion of clutch hub 80.

To provide means for selectively controlling the magnitude of the clutchengagement force exerted on reaction plate 112, rotary actuator means 40is associated with a mechanical drive mechanism 114 and includes anelectrically-controlled rotary actuator 116. Preferably, rotary actuator116 is an electric gearmotor which is operable for generating an outputtorque, the value of which varies as a function of the magnitude of theelectrical control signal applied thereto by controller means 46. Ingeneral, drive mechanism 114 is interconnected to a rotary output member118 of rotary actuator 116 for changing the output torque into anaxially-directed force for controlling the clutch engagement forceapplied to reaction plate 112 of transfer clutch 38. As best seen fromFIGS. 3 and 4, drive mechanism 114 includes a sector plate 120 that isrotatably driven through a limited range of angular motion by outputmember 118 of rotary actuator 116 and a lever arm assembly 122. Sectorplate 120 is interconnected to lever arm assembly 122 which is adaptedto exert the clutch engagement force on reaction plate 112 in responseto controlled rotation of sector plate 120. A rotatable actuating shaft124 is supported from transfer case housing 56 for rotation about anaxis 126. A first end of actuating shaft 124 is secured in an aperture128 formed in sector plate 120, with its opposite end coupled to outputmember 118 of electrically-controlled rotary actuator 116. Thus,actuating shaft 124 and sector plate 120 are rotated about axis 126 inresponse to rotation of output member 118 upon actuation of rotaryactuator 116.

To control the magnitude of the clutch engagement force exerted onreaction plate 112, sector plate 120 includes a contoured mode slot 130into which a crowned roller 132 extends. Crowned roller 132 is fixed toa flange section 134 of a lever member 136 associated with lever armassembly 122. Lever member 136 also includes a generally Y-shaped orforked section 138 which is bifurcated to surround mainshaft 64 andclutch hub 80. The bifurcated ends of forked section 138 are retainedfor pivotal movement on a transverse rail 140, the ends of which areretained in suitable sockets (not shown) formed in housing 56. Ingeneral, the contour of mode slot 130 is configured to cause pivotablemovement of lever member 136 in response to rotation of sector plate 120for controlling the clutch engagement force exerted on reaction plate112 of transfer clutch 38. Moreover, a plurality ofcircumferentially-spaced buttons 142 are secured to a front surface offorked section 138 and are adapted to apply the clutch engagement forceto reaction plate 112 for compressing interleaved clutch plates 108 and110 via a suitable thrust mechanism. Preferably, the thrust mechanismincludes an annular apply plate 144 that is journally supported onclutch hub 80 and positioned intermediate reaction plate 112 and levermember 136, and a thrust bearing/washer arrangement 146 interposedbetween apply plate 144 and reaction plate 112 for allowing rotation ofreaction plate 112 with respect to apply plate 144.

With particular reference to FIG. 4, means are provided for coordinatingthe pivotal movement of lever arm assembly 122 upon rotation of sectorplate 120 between the two distinct sector positions, as labelled "4WD"and "2WD". In general, the contour of mode slot 130 is defined by a campathway 148. In the 4WD sector position shown, crowned roller 132 ispositioned within mode slot 130 in close proximity to the terminal endof cam pathway 148 for establishing a four-wheel drive (4WD) position.With crowned roller 132 in the four-wheel drive (4WD) position, leverarm assembly 122 exerts a maximum clutch engagement force on reactionplate 112 such that transfer clutch 38 is considered to be operating ina fully "actuated" condition. In this fully actuated condition,mainshaft 64 is effectively coupled to front output 88 due to themaximum torque delivered through transfer clutch 38.

As sector plate 120 is caused to rotate about axis 126 in a firstdirection (i.e., counterclockwise) from the position shown, the contourof cam pathway 148 causes axial displacement of crowned roller 132toward a two-wheel drive (2WD) position. Such movement of crowned roller132 causes concurrent pivotable movement of lever arm assembly 122 whichresults in a proportional decrease in the clutch engagement force thatis being exerted on reaction plate 112. Moreover, once crowned roller132 is in the two-wheel drive (2WD) position, lever arm assembly 122does not exert an engagement force on reaction plate 112 which issufficient to transfer drive torque through transfer clutch 38 to clutchhub 80, whereby transfer clutch 38 is considered to be in a"non-actuated" condition. As will be appreciated, rotation of sectorplate 120 in the opposite direction (i.e., clockwise) from the 2WDsector position toward the 4WD sector position results in movement ofcrowned roller 132 toward the four-wheel drive (4WD) position, wherebylever arm assembly 122 is pivoted about rail 140 for proportionallyincreasing the clutch engagement force exerted on reaction plate 112.

According to the embodiment disclosed, rotary actuator 116 is actuatedin accordance with specific predefined relationships that areestablished in response to the current value of the sensor input signalsfor rotatably driving sector plate 120 to any position between the 2WDand 4WD sector positions. Thus, the amount of torque transferred"on-demand" through transfer clutch 38 is proportional to the clutchengagement force, the value of which is determined by the particularposition of crowned roller 132 between the two-wheel drive (2WD)position and the four-wheel drive (4WD) position that is establishedupon controlled rotation of sector plate 120.

In its most basic sense, power transfer system 10 automatically andinstantaneously transfers drive torque "on-demand" to front wheels 12during the occurrence of slippage of rear wheel 14 that are typicallyassociated with low tractive road conditions. In addition, powertransfer system 10 functions to continuously monitor and regulate the"on-demand" operation in a manner that is independent of any deliberateaction by the vehicle operator. Accordingly, the modulation range isestablished between the limits of bi-directional sector rotation asdefined by movement of crowned roller 132 within cam pathway 148 betweenthe (2WD) and (4WD) positions. Moreover, the magnitude of the clutchengagement force generated by lever arm assembly 122 and applied totransfer clutch 38 is proportional to the magnitude of the output torquegenerated by rotary actuator 116 which, in turn, is proportional to themagnitude of the control signal (i.e., percentage duty cycle) applied bycontroller means 46 to rotary actuator 116. Thus, the amount of drivetorque transferred through transfer clutch 38 to front output shaft 88is also proportional to the magnitude of the control signal. As such,the distribution ratio of drive torque between front output shaft 88 andmainshaft 64 of transfer case 20 may be selectively varied as a functionof changes in the magnitude of the control signal for optimizing thetractive performance characteristics.

With particular reference now to FIG. 5, a block diagram of a controlsystem associated with power transfer system 10 is shown. Preferably,controller means 46 is an electronic control module 150 having signalprocessing and information storage capabilities. In addition, firstsensor means 42 is shown as a group of various "system" sensors that areprovided for detecting and signaling specific dynamic and operationalcharacteristics of the motor vehicle. The input signals generated by the"systems" sensor group are delivered to electronic control module 150.Preferably, these sensors include a front speed sensor 152 for sensingthe rotational speed (n_(F)) of front drive shaft 34, a rear speedsensor 154 for sensing the rotational speed (n_(R)) of rear drive shaft28, a vehicle speed sensor 156 for sensing a vehicle speed (V), anignition switch 158 for signalling the operational status of thevehicle, and a battery input 160 for powering electronic control module150. In vehicles equipped with an anti-lock brake system (ABS), theoriginal equipment ABS sensors provided at each wheel can be used fordetermining an "average" front drive shaft speed and rear drive shaftspeed. Alternatively, front and rear speed sensors 152 and 154,respectively, can be arranged for directly measuring the speed of frontoutput shaft 88 and mainshaft 64, respectively. Moreover, it is possiblefor vehicle speed sensor 156 to be eliminated with the vehicle speedsignal (V) being computed from the front rotational speed signal (n_(F))generated by front speed sensor 152. However, it is to be understoodthat any suitable speed sensing arrangement capable of generating asignal indicative of the rotational speed of a shaft is fairly withinthe scope of the present invention.

The control system also illustrates the use of various"operator-initiated" inputs, as generally categorized by second sensormeans 50. These inputs include a brake sensor 162 for sensing when thevehicle operator is applying the brakes, a steering angle sensor 164 fordetecting the magnitude of a steering angle (Φ), and an acceleratorsensor for sensing an accelerating condition of the vehicle. Preferably,the accelerator sensor is a throttle position sensor 166 for sensing thedegree of opening of a throttle valve associated with engine 16 or forsensing the degree of depression of an accelerator pedal, and isoperable to produce a throttle position signal (A). Theoperator-initiated input signals are delivered to control module 150where they are used, in conjunction with the system input signals, tofurther control "on-demand" operation.

With reference now to FIG. 6, a control sequence for automaticallycontrolling the "on-demand" operation of power transfer system 10 isshown. In general, the flow chart represents a sequence of theoperations performed by electronic control module 150 and which arediagrammatically shown in block form. More specifically, the flow chartillustrates a succession of control steps are continuously repeated forselecting the value of the control signal to be applied to rotaryactuator 116 in accordance with various predefined relationships betweenthe current value of a front and rear wheel speed differential (ΔN) andvehicle speed GO, as modified by the steering angle (Φ) and otheroperator-initiated inputs. Block 168 is representative of the controlstep in which the current value of the rotational speed of front driveshaft 34 (n_(F)) and rear drive shaft 28 (n_(R)) are read. Block 170indicates the step of reading the value of steering angle (Φ) asdetected by steering angle sensor 164. Block 172 represents theoperation of selecting a control characteristic (C) in accordance withthe steering angle (Φ). FIG. 8 illustrates a plot of an exemplaryrelationship, which may be stored as a look-up table or computed from anarithmetic equation in control module 150, which correlates the controlcharacteristic (C) as a linear function of the detected steering angle(Φ). Next, block 174 represents the step of calculating a speeddifferential (ΔN) according to the equation

    ΔN=n.sub.R -n.sub.F +C

Blocks 176 and 178 indicate the steps of reading the current value ofthe vehicle speed (V) as detected by vehicle speed sensor 156 and thethrottle position (A) as detected by throttle position sensor 166,respectively. As shown in block 180, control module 150 determineswhether the vehicle speed (V) exceeds a predefined threshold value(V_(T)) such as, for example, 5 mph. If the vehicle speed is less thanthe threshold value (V_(T)), a second determination is made (block 182)as to whether the value of the throttle position (A) exceeds apredefined threshold value (A_(T)) such as, for example, a 50%accelerator pedal depression angle. If the vehicle speed (V) is lessthan its threshold value (V_(T)) and the throttle position (A) exceedsits threshold value (A_(T)), then the magnitude (i.e., percentage ofduty cycle) of the electrical control signal is set as a preset value,such as 30% duty cycle, as indicated by block 184. In this manner, powertransfer system 10 is adapted to transfer torque to front wheels 12 inresponse to acceleration at low vehicle speeds to inhibit wheel slip.However, if the value of the throttle position (A) is less than itsthreshold value (A_(T)), then the magnitude of the duty cycle for thecontrol signal is set in accordance with predefined relationshipsbetween the speed differential signal (ΔN) and vehicle speed (V), asindicated by block 186. Block 188 represents the step of outputting theelectrical control signal to rotary actuator 116 for developing thedesired amount of torque transfer, if any, across transfer clutch 38. Asshown in block 190, a timer circuit within control module 150 isactuated simultaneously with energization of actuator 116 formaintaining such energization for a predetermined time period (T). Oncethe period of energization (t) equals the predetermined time period (T)(or t≦T), control module 150 repeats the control routine.

To enhance steering control, block 192 is indicative of the control stepbetween steps 180 and 186 for determining whether the vehicle operatoris applying the brakes when the vehicle speed (V) is greater than thethreshold value (V_(T)). Accordingly, if the vehicle operator isattempting to stop the vehicle, by applying the brakes (as sensed bybrake sensor 162) during an occurrence of a low traction road conditionand the vehicle speed (V) is greater than the predefined threshold(V_(T)), then control module 150 sets the magnitude of the controlsignal sent to rotary actuator 116 to zero (block 194) for de-actuatingtransfer clutch 38 and disabling the "on-demand" feature. This controlsequence prevents simultaneous braking and "on-demand" four-wheeloperation for providing the vehicle operator with greater steering andbraking control. However, during the occurrence of a low tractioncondition when brake sensor 162 signals control module 150 that thevehicle operator is not applying the brakes, electronic control module150 automatically energizes rotary actuator 116 (block 188) foractuating transfer clutch 38 in accordance with the relationshipsgenerally denoted by block 186.

With particular reference to FIG. 7, a set of exemplary plots used forestablishing the magnitude of the duty cycle to be sent to rotaryactuator 116 in response to the current value of the speed differential(ΔN) and vehicle speed (V) during "on-demand" operation, asdiagrammatically referred to by block 186 in FIG. 6, will now bedetailed. As seen, power transfer system 10 linearly correlates thepercentage duty cycle of the control signal applied to rotary actuator116 to a range of speed differential (ΔN) values. In general, thepercentage duty cycle for the control signal increases linearly from aminimum actuation value (Y%) to a maximum actuation value (100%) as thevalue of the speed differential (ΔN), within a particular vehicle speedrange, increases from a minimum speed differential limit to a maximumspeed differential limit (X). As such, when the value of the speeddifferential (ΔN) is less than the minimum speed differential limit, nodrive torque is transmitted through transfer clutch 38 to front outputshaft 88. However, when the value of the speed differential (ΔN) exceedsthe minimum differential limit, "on-demand" four-wheel drive operationis established by supplying a control signal to rotary actuator 116having a duty cycle value greater than (Y%). Thus, the minimum actuationduty cycle (Y%) for the control signal correlates to the point at whichfrictional engagement between interleaved clutch plates 108 and 110results in the delivery of a portion of the total drive torque to frontoutput shaft 88 of transfer case 20 for initiating "on-demand"four-wheel drive operation.

The portion of the total drive torque transferred through transferclutch 38 to front output shaft 88 increases substantially linearly asthe magnitude of the duty cycle for the control signal increases fromthe minimum actuation value (Y%) to the maximum actuation value (100%).Preferably, the maximum value (X) of the speed differential (ΔN)correlates to the maximum actuation duty cycle (100%) at which point themaximum clutch engagement force is generated for completely locking-upclutch plates 108 and 110. During "on-demand" four-wheel driveoperation, a reduction in the magnitude of the control signal sent torotary actuator 116 will result in actuator output member 118 beingback-driven due to the clutch engagement load exerted by lever armassembly 122 on sector plate 120. As such, a zero control signal willback-drive sector plate 120 until crowned roller 132 is in the two-wheeldrive (2WD) position. Alternatively, the direction of driven rotation ofactuator output member 118 may be reversed until the desired clutchengagement force is established. As best seen from FIG. 9, an exemplarylinear relationship between the magnitude of the duty cycle supplied torotary actuator 116 and the clutch engagement force generated and, inturn, the amount of torque delivered across transfer clutch 38 is shown.

In accordance with an alternative embodiment of the present invention,power transfer system 10 is also equipped with mode select means 44 forestablishing at least three distinct operational modes, namely atwo-wheel drive mode, a part-time four-wheel drive mode, and an"on-demand" drive mode. In operation, the vehicle operator selects thedesired mode via mode select means 44 which, in turn, signals controllermeans 46 of the selection. Thereafter, controller means 46 generates anelectrical control signal that is applied to rotary actuator 116 forcontrolling the rotated position of sector plate 120. To provide meansfor the vehicle operator to shift power transfer system 10 into one ofthe available operational modes, mode select means 44 can take the formof any mode selector device which is under the control of the vehicleoperator for generating a mode signal indicative of the specific modeselected. In one form, the mode selector device may be an array ofdash-mounted push button switches. Alternatively, the mode selectordevice may be a manually-operable shift lever sequentially movablebetween a plurality positions corresponding to the available operationalmodes which, in conjunction with a suitable electrical switcharrangement, generates a mode signal indicating the mode selected. Ineither form, mode select means 44 offers the vehicle operator the optionof deliberately choosing between the part-time and on-demand operativedrive modes.

With reference now to FIG. 10, a control sequence for the selection andthe subsequent automatic control of the "on-demand" drive mode is shown.In general, the flow chart is identical to that shown in FIG. 6 with theaddition of control steps for integrating mode select means 44 into thecontrol system. When mode select means 44 signals selection of the"on-demand" mode, as indicated by block 200, a succession of controlsteps are continuously repeated for selecting the value of the controlsignal to be applied to rotary actuator 116 in accordance with theabove-noted predefined relationships between the current value of afront and rear wheel speed differential (ΔN) and vehicle speed (V), asmodified by the steering angle (Φ) and other operator-initiated inputs.However, if any other mode is selected, then the control sequence jumpsto a part-time routine, as indicated by block 202. When the vehicleoperator selects an operational mode via mode select means 44 other thanthe "on-demand" drive mode, control module 150 controls the energizedcondition of rotary actuator 116 for rotating sector plate 120 into oneof the 2WD or 4WD sector positions which corresponds to the two-wheeldrive mode or part-time four-wheel drive mode, respectively. Moreparticularly, if the two-wheel drive mode is selected, control module150 sends an electrical control signal to rotary actuator 116 forrotating sector plate 120 in the first direction to the 2WD sectorposition for causing movement of crowned roller 132 to its two-wheeldrive (2WD) position. If the part-time four-wheel drive mode isselected, then rotary actuator 116 is fully actuated to rotate sectorplate 120 in the opposite direction to the 4WD sector position formoving crowned roller 132 to its four-wheel drive (4WD) position.

With reference now to FIG. 11, an alternative construction for a drivemechanism 204 is shown which is generally directed to replace lever armassembly 122 of drive mechanism 114. As such, like numbers are used toidentify those components previously described. In general, drivemechanism 204 includes sector plate 120 (FIG. 4) and an axially movablemode sleeve 206 which is journally supported for limited axial slidingmovement on clutch hub 80 and positioned intermediate reaction plate 112and drive sprocket 76. In addition, mode sleeve 206 has a front facesurface 208 which is adapted to apply the clutch engagement force toreaction plate 112. A fork assembly 210 couples mode sleeve 206 tosector plate 120 for changing the output torque of rotary output member118 into an axially-directed force for controlling the clutch engagementforce exerted by face surface 208 on reaction plate 112.

Shift fork assembly 210 includes a shift rail 212 retained for slidingmovement in sockets 214 and 216 formed in housing 56, and a shift fork218 fixed to shift rail 212 and having a bifurcated fork portion 220retained within a annular groove 222 in mode sleeve 206. Crowned roller132 is fixed, via pin 224 to a tubular portion 226 of shift fork 218 forcoupling shift fork 218 for axial sliding movement with rail 212. Asbefore, crowned roller 132 extends into contoured mode slot 130 formedin sector plate 120, wherein the contour of mode slot 130 is configuredto cause axial movement of shift fork assembly 210 and mode sleeve 206in response to rotation of sector plate 120 for controlling the clutchengagement force exerted on reaction plate 112 of transfer clutch 38. Inthe 4WD sector position, crowned roller 132 is positioned within modeslot 130 in close proximity to the terminal end of cam pathway 148 foragain establishing the four-wheel drive (4WD) position. With crownedroller 132 in the four-wheel drive (4WD) position, face surface 208 ofmode sleeve. 206 exerts a maximum clutch engagement force on reactionplate 112 such that transfer clutch 38 is considered to be operating ina fully actuated condition. As sector plate 120 is caused to rotateabout axis 126 in the first direction from the 4WD sector position, thecontour of cam pathway 148 causes axial displacement of crowned roller132 toward the two-wheel drive (2WD) position. Such movement of crownedroller 132 causes concurrent axial movement of fork assembly 210 andmode sleeve 206 which results in a proportional decrease in the clutchengagement force that is being exerted on reaction plate 112. Thus, theamount of torque transferred through transfer clutch 38 is proportionalto the clutch engagement force, the value of which is determined by theparticular position of crowned roller 132 between the two-wheel drive(2WD) position and the four-wheel drive (4WD) position that isestablished upon controlled rotation of sector plate 120.

With particular reference now to FIGS. 12 through 14, anotheralternative construction is shown for an electronically-controlledtorque-modulatable transfer case, hereinafter designated by referencenumeral 300. Transfer case 300 can be incorporated into the drivelinearrangement shown in FIG. 1 for operation pursuant to the control formatand characteristic relationships set forth in FIGS. 5 through 10.Accordingly, since the actuation and control of transfer case 300 isgenerally similar to that previously disclosed, like numbers are used todesignate components thereof that are identical or substantially similarin structure and/or function to those disclosed relative to transfercase 20.

Transfer case 300 is adapted for incorporation into power transfersystem 10 and includes an electronically-controlled torque transferarrangement for transmitting drive torque to front wheels 12 in additionto rear wheels 14 for establishing the part-time and on-demandfour-wheel drive modes. The torque transfer arrangement includes atransfer clutch 302 that is operable for transferring drive torque fromfirst output member 30 to second output member 36, thereby deliveringdrive torque to front wheels 12. In a system equipped with mode selectmeans 44, rotary actuator means 40 is again operable for actuatingtransfer clutch 302 in response to a mode signal generated by thevehicle operator. When a two-wheel drive mode is available and selected,all drive torque is delivered from first output member 30 to rear wheels14 and transfer clutch 302 is maintained in a "non-actuated" condition.When a part-time four-wheel drive mode is available and selected,transfer clutch 302 is fully actuated and maintained in a "lock-up"condition such that second output member 36 is, in effect, rigidlycoupled for driven rotation with first output member 30. When transfercase 300 is operating in an "on-demand" drive mode, the amount of drivetorque directed to front wheels 12 through transfer clutch 302 isautomatically modulated as a function of various sensor signals forproviding enhanced traction when needed.

With continued reference to FIGS. 12 through 14, the preferredconstruction for transfer case 300 will now be described with greaterspecificity. Transfer case 300 is shown to include housing 56 formed bya series of modular sections that are suitably interconnected in aconventional manner. Transmission output shaft 58 couples transmission18 to input shaft 60 of transfer case 300 for supplying power thereto.Input shaft 60 has annular input gear 62 formed integral therewith. Inthe embodiment shown, first output member 30 is an elongated mainshaft304 which is aligned on the longitudinal axis of input shaft 60 and issupported for rotation within housing 56. A sleeve 306 is concentricallysupported on a forward end portion of mainshaft 304 and is fixed forrotation therewith by means of internal splines 308 engaged withcorresponding sets of external splines 310 formed on mainshaft 304. Inaddition, sleeve 306 is formed with external clutch teeth 312 that areshown to be meshingly engaged with internal clutch teeth 74 formed oninput gear 62. A snap ring is provided for locating and axiallyretaining sleeve 306 on mainshaft 304 relative to input gear 62. Withthis arrangement, drive torque is transferred without reduction frominput shaft 60 to mainshaft 304.

As best seen from FIGS. 12 and 13, transfer clutch 302 is operablyinstalled within transfer case 300 for selectively transferring drivetorque from mainshaft 304 to front output shaft 88. Transfer clutch 302is a mechanically-actuated multi-plate clutch assembly that is arrangedto concentrically surround a portion of mainshaft 304. According to theparticular construction shown, transfer clutch 302 includes a drivesprocket 76 that is fixed (i.e., splined) for rotation with an outerdrum 316. As seen, outer drum 316 is supported on mainshaft 304 forrotation relative thereto by a suitable bearing assembly 318. Drivesprocket 76 drivingly engages chain 84 which is coupled to lower drivensprocket 86. As previously noted, driven sprocket 86 is coupled to, oran integral portion of, second output member 36 which is shown as frontoutput shaft 88. As also noted, front output shaft 88 is operablyconnected to the motor vehicle's front wheel 12 via front drive shaft34. Thus, outer drum 316 is fixedly secured to drive sprocket 76 so asto drive, or be driven by, front output shaft 88 of transfer case 300.

Transfer clutch 302 also includes an inner drum 320 that is fixed (i.e.,splined) to mainshaft 304 for rotation therewith. In addition, outerdrum 316 is arranged to concentrically surround inner drum 320 so as toform an internal chamber 332 therebetween. Thus, outer drum 31 6 andinner drum 320 are capable of rotating relative to one another. Innerdrum 320 is shown as an integral component having an annular hub 322splined to mainshaft 304, a web 324 extending radially from annular hub322, and a cylindrical drum 326 formed at the opposite end of web 324and extending coaxially to hub 322. Outer drum 316 has a cylindricaldrum 328 which is enclosed at one end by a cover plate 330. As seen,cover plate 330 includes an integral tubular extension 314 that issupported on bearing assembly 318.

Disposed within internal chamber 332 are two sets of alternatelyinterleaved friction clutch plates that are operable for transferringdrive torque from mainshaft 304 and inner drum 320 to outer drum 316 anddrive sprocket 76 so as to ultimately deliver drive torque to frontoutput shaft 88 in response to a clutch engagement force applied to theclutch plates. One set of clutch plates, referred to as inner clutchplates 108, are mounted (i.e., splined) to an outer peripheral surfaceof cylindrical drum 326 for driven rotation with mainshaft 304. Thesecond set of clutch plates, referred to as outer clutch plates 110, aremounted (i.e., splined) to an inner peripheral surface of cylindricaldrum 328 for rotation with drive sprocket 76. In addition to innerclutch plates 108, a component of a sliding thrust mechanism 334 ismounted on cylindrical drum 326 of inner drum 320 for rotation therewithand axial movement with respect thereto. As will be described, thrustmechanism 334 is slidably movable on mainshaft 304 and is operable forfrictionally compressing the interleaved clutch plates so as to causedrive torque to be transferred through transfer clutch 302 as a functionof the clutch engagement force exerted thereon.

To provide means for selectively controlling the magnitude of the clutchengagement force exerted on thrust mechanism 334, rotary actuator means40 is associated with a mechanical drive mechanism 336 and includes anelectrically-controlled rotary actuator 116. As noted, rotary actuator116 is an electric gearmotor which is operable for generating an outputtorque, the value of which varies as a function of the magnitude of theelectrical control signal applied thereto by controller means 46. Drivemechanism 336 is interconnected to a rotary output member 118 of rotaryactuator 116 for changing the output torque into an axially-directedforce used for controlling the clutch engagement force applied to thrustmechanism 334 of transfer clutch 302. To this end, drive mechanism 336includes a pivotable lever arm assembly 122 and a sector plate 338 thatis rotatably driven through a limited range of angular motion by outputmember 118 of rotary actuator 116. More specifically, rotation of sectorplate 338 is adapted to cause pivotable movement of lever arm assembly122 which, in turn, causes sliding movement of thrust mechanism 334 forexerting the clutch engagement force on the interleaved clutch plates.

To generate the desired clutch engagement force, sector plate 338includes a contoured peripheral edge 340 against which a crowned roller132 rests. As noted, crowned roller 132 is fixed to a flange section 134of a pivotable lever member 136 associated with lever arm assembly 122.Lever member 136 includes a forked section 138 which is bifurcated tosurround mainshaft 304. The bifurcated ends of forked section 138 areretained for pivotal movement on transverse rail 140, the ends of whichare retained in suitable sockets (not shown) formed in housing 56. Ingeneral, the contour of sector edge 340 is configured to cause pivotablemovement of lever member 136 in response to rotation of sector plate 338for controlling the clutch engagement force exerted on thrust mechanism334 of transfer clutch 302.

Thrust mechanism 334 includes an annular inner bearing support 342journally supported for sliding non-rotatable movement on mainshaft 304.While not shown, inner bearing support 342 includes an axial tang whichis nested within a corresponding aperture in lever arm 136 forinhibiting rotation of inner bearing support 342 relative to mainshaft304 and inner drum 320. Thrust mechanism 334 also includes an annularouter bearing support 344 that is coupled for rotation with inner drum320. In particular, outer bearing support 344 includes a tubular segment346 supported for sliding axial movement relative to cylindrical hub 326and a radial plate segment 348 which acts as a pressure plate forfrictionally compressing the interleaved clutch plates. As is also seen,a series of apertures 350 are formed in plate segment 348 of outerbearing support 344. Axial lugs 352 formed on the distal end ofcylindrical drum 326 are nested within apertures 350 for coupling outerbearing support 344 for rotation with, and axial movement relative to,inner drum 320. Thus, outer bearing support 344 is supported forrotation with inner drum 320 and mainshaft 304 while inner bearingsupport 342 is held stationary relative thereto. A thrust bearingassembly 354 is mounted between inner bearing support 342 and outerbearing support 344 for facilitating such relative rotation therebetweenwhile accommodating the thrust forces exerted on thrust mechanism 334. Aseries of buttons 142 mounted to lever arm 136 act on inner bearingsupport 342 for causing sliding movement of the entire thrust mechanism334 in response to pivotable movement of lever arm assembly 122 forcausing the clutch engagement force to be exerted by plate segment 348of outer bearing support 344 on the interleaved clutch plates. Finally,an annular return spring 356 is retained between inner drum 320 andouter bearing support 344 for normally biasing sliding thrust mechanism334 toward the clutch "non-actuated" condition.

With particular reference to FIG. 14, the means associated with drivemechanism 336 for establishing the range of pivotal movement of leverarm assembly 122 that is generated in response to rotation of sectorplate 338 between two distinct sector positions will now be described.As stated, the specific contour of sector edge 340 is adapted to causeaxial movement of crowned roller 132 upon rotation of sector 338. In the2WD sector position shown, crowned roller 132 is positioned against edge340 in close proximity to a terminal end 356 of an arcuate cam pathway358 for establishing a two-wheel drive (2WD) roller position. Withcrowned roller 132 in the two-wheel drive (2WD) position, lever armassembly 122 exerts a minimal clutch engagement force on thrustmechanism 334 such that transfer clutch 302 is considered to beoperating in a "non-actuated" condition. In this non-actuated condition,transfer clutch 302 does not transfer drive torque from mainshaft 304 tofront output 88.

As sector plate 338 is caused to rotate about axis 126 in a firstdirection (i.e., clockwise) from the position shown, the contour of campathway 358 causes axial displacement of crowned roller 132 toward afour-wheel drive (4WD) position on cam pathway 358, as indicated at 360.Such movement of crowned roller 132 causes concurrent pivotable movementof lever arm assembly 122 toward transfer clutch 302 which results in aproportional increase in the clutch engagement force that is exerted onclutch plates 108 and 110 by thrust mechanism 334. Moreover, oncecrowned roller 132 is moved axially to the four-wheel drive (4WD)position, lever arm assembly 122 exerts a maximum clutch engagementforce on thrust mechanism 334, whereby transfer clutch 302 is consideredto be in its fully "actuated" condition. As will be appreciated,rotation of sector plate 338 in the opposite direction (i.e.,counterclockwise) from the 4WD sector position toward the 2WD sectorposition results in movement of crowned roller 132 toward its two-wheeldrive (2WD) position, whereby lever arm assembly 122 is pivoted awayfrom transfer clutch 302 for proportionally decreasing the clutchengagement force exerted on thrust mechanism 334.

During "on-demand" operation, a power transfer system equipped withtransfer case 300 functions to continuously monitor and regulate thetorque transfer characteristics in a manner that is independent of anydeliberate action by the vehicle operator. As noted, the amount oftorque transferred through transfer clutch 302 is proportional to theclutch engagement force, the value of which is determined by theparticular position of crowned roller 132 on cam pathway 358 between itstwo-wheel drive (2WD) and four-wheel drive (4WD) positions due tocontrolled rotation of sector plate 338. Accordingly, the modulationrange is established between the limits of bi-directional sectorrotation as defined by movement of crowned roller 132 against campathway 358 between the (2WD) and (4WD) roller positions. Moreover,since the magnitude of the clutch engagement force generated by leverarm assembly 122 and applied to transfer clutch 302 is proportional tothe magnitude of the output torque generated by rotary actuator 116which, in turn, is proportional to the magnitude of the control signal(i.e., percentage duty cycle) applied by controller means 46 to rotaryactuator 116, the amount of drive torque transferred through transferclutch 302 to front output shaft 88 is also proportional to themagnitude of the control signal. As such, the distribution ratio ofdrive torque between front output shaft 88 and mainshaft 304 of transfercase 300 may be selectively varied as a function of changes in themagnitude of the control signal for optimizing the tractive performancecharacteristics. Preferably, control parameters and relationshipssimilar to those set forth in FIGS. 5 through 9 are applicable whentransfer case 300 is used in continuous "on-demand" power transfersystems while controls similar to that shown in FIG. 10 are applicableto systems equipped with mode select means 44. More preferably, the"on-demand" control schemes set forth in FIGS. 6 and 10 are slightlymodified to eliminate use of an input signal from steering angle sensor164 (FIG. 5). As such, the control constant (C) is not used incalculating the current value of the speed differential (ΔN) which, inturn, is used for controlling the modulated control of transfer clutch302. As noted, mode select means 44 can take the form of any modeselector device which is under the control of the vehicle operator forgenerating a mode signal indicative of the specific mode selected tooffer the vehicle operator the option of deliberately choosing betweenat least one of the part-time drive modes and the on-demand drive mode.

With reference now to FIGS. 15 and 16, a modified construction oftransfer case 300, hereinafter designated by reference numeral 400, isshown. In general, transfer case 400 includes means for permitting thevehicle operator to selectively disconnect mainshaft 304 from inputshaft 60 for establishing a non-driven or "Neutral" mode. Thus, transfercase 400 is particularly well-suited for incorporation into powertransfer systems equipped with mode select means 44 for establishing theNeutral mode in addition to the "on-demand" drive mode and one or moreof the two-wheel drive and part-time four-wheel drive modes. Due tosimilarity of components, like numbers are used hereinafter to identifythose components that are identical to or similar in structure and/orfunction to those previously described.

To provide means for selectively coupling and de-coupling mainshaft 304with respect to input shaft 60, a shift sleeve 402 is supported forrotation with and axial sliding movement on mainshaft 304 due to theengagement of internal splines 404 with external splines 310 onmainshaft 304. In addition, shift sleeve 402 is formed with externalclutch teeth 406 that are shown meshingly engaged with clutch teeth 74formed on input gear 62. In this "coupled" position, drive torque istransferred from input shaft 60 through shift sleeve 402 to mainshaft304 for establishing a drive connection through transfer case 400.Accordingly, construction line "D" identifies the position of shiftsleeve 402 when such a "Drive" mode is established. However, when shiftsleeve 402 is slid rearwardly to a "de-coupled" position wherein itsclutch teeth 406 disengage clutch teeth 74 on input gear 62, then nodrive torque is transmitted from input shaft 60 to mainshaft 304 and nopower is transmitted through transfer case 400 to the vehicle's rearwheels 14. Accordingly, construction line "N" identifies the position ofshift sleeve 402 when a non-driven "Neutral" mode is established. Such aprovision for a Neutral mode arrangement is particularly desireable forflat towing (all four wheels on the ground) of the motor vehicle whichcan be accomplished without requiring disassembly of the front or reardrivelines.

Axial sliding movement of shift sleeve 402 between the two distinct (D)and (N) positions is caused by axial movement of a shift fork 408. Aswill be described, such movement of shift fork 408 is controlled byactuator means 40 in response to the mode signal delivered to controllermeans 46 via mode select means 44. In particular, a drive mechanism 410is used in association with rotary actuator 116 for selectively movingshift sleeve 402 between the (D) and (N) positions while concurrentlycontrolling the magnitude of the clutch engagement force exerted on theinterleaved clutch plates. Drive mechanism 410 is generally similar todrive mechanism 336 (FIGS. 12 through 14) with the exception thatprovisions have been made for selectively controlling movement of shiftsleeve 402 in coordination with controlled actuation of transfer clutch302.

Drive mechanism 410 includes a sector plate 412 that is rotatably driventhrough a limited range of angular motion by output member 118 of rotaryactuator 116 for causing pivotable movement of lever arm assembly 122which, in turn, controls the magnitude of the clutch engagement forceexerted by thrust mechanism 334 on the clutch pack. In addition, sectorplate 412 is adapted to concurrently control the axial position of shiftfork 408 and, in turn, shift sleeve 402 in response to such controlledrotation of sector plate 412. As best seen from FIG. 15, shift fork 408is coupled to a spring-loaded shift fork assembly 414 that is supportedfor sliding movement on a shift rail 416 and which is generally similarto that described in commonly owned U.S. Pat. No. 4,529,080 to Dolan,the disclosure of which is expressly incorporated by reference herein.It can also be seen that a range pin 418 is fixed to a U-shaped bracket420 of shift fork assembly 414 which, in turn, is retained for slidingmovement on shift rail 416. In addition, shift fork 408 is coupled tobracket 420 for movement therewith. While not directed to the novelty ofthis invention, shift fork assembly 414 includes a spring-biasedarrangement adapted to normally bias shift fork 408 and shift sleeve 402toward the drive (D) position to assist in completing meshed engagementof clutch teeth 406 on sleeve 402 with input gear teeth 74 during aNeutral mode to Drive mode shift.

From FIG. 16, it can be seen that sector plate 412 may be rotated aboutaxis 126 by rotary actuator 116 to any of three distinct sectorpositions, as labelled "4WD", "2WD" and "N". To control movement ofshift sleeve 402, sector plate 412 has an elongated range slot 422formed therein into which range pin 418 extends. The contour of rangeslot 422 is configured to cause the desired translational movement ofbracket 420, shift fork 408 and shift sleeve 402 in response tocontrolled bi-directional rotation of sector plate 412. Moreover, inview of incorporation of shift sleeve 402 into transfer case 400, thepower transfer system is capable of establishing at least four distinctoperative modes, namely a two-wheel drive mode, a part-time four-wheeldrive mode, an on-demand drive mode and a Neutral mode. As will bedescribed, the particular mode selected is established by the positionof crowned roller 132 against contoured sector edge 340 and the positionof range pin 418 within range slot 422, as concurrently established inresponse to the rotated position of sector plate 412. In operation, thevehicle operator selects the desired operative mode via mode selectmeans 44 which, in turn, signals controller means 46 of the selection.Thereafter, controller means 46 generates an electrical control signalthat is applied to actuator 116 for controlling the rotated position ofsector plate 412. Moreover, for each of the two-wheel drive, part-timefour-wheel drive and Neutral modes, sector plate 412 is controllablyrotated to its corresponding 2WD, 4WD and N sector position. However,when the on-demand drive mode is selected, power transfer system 10 isoperable for modulating the clutch engagement force applied to transferclutch 302 as a function of various system and operator initiated inputsin the manner previously disclosed.

With continued reference to FIG. 16, means are shown for coordinatingthe axial movement of shift fork assembly 414 and the pivotable movementof lever arm assembly 122 upon rotation of sector plate 412 between thevarious sector positions for establishing the desired combination ofdrive modes. In general, the contour of range slot 422 is defined by afirst guideway 424 and a second guideway 426 which respectivelycorrespond to first and second cam pathways 358 and 428, respectively,that are sequentially formed on contoured edge 340. In the 2WD sectorposition shown, crowned roller 132 is positioned on first cam pathway358 at point 356 for establishing a two-wheel drive (2WD) rollerposition. As previously noted, with crowned roller in its two-wheeldrive (2WD) position, lever arm assembly 122 does not exert a sufficientclutch engagement force on thrust mechanism 334 to transfer drive torquethrough transfer clutch 302, whereby transfer clutch 302 is consideredto be in its "non-actuated" condition. Concurrently, range pin 41 8 isshown in position within range slot 422 in close proximity to one end offirst guideway 424 for axially locating shift sleeve 402 in the drive(D) position.

As sector plate 412 is caused to rotate about axis 126 in a first (i.e.,clockwise) direction from the position shown, the contour of first campathway 358 causes axial displacement of crowned roller 132 toward afour-wheel drive (4WD) position. Such axial movement of crown roller 132causes concurrent pivotable movement of lever arm 136 which results in aproportional increase in the clutch engagement force exerted on thrustmechanism 334. With crowned roller 132 in its four-wheel drive (4WD)position, lever arm assembly 122 exerts a maximum clutch engagementforce on thrust mechanism 334 such that transfer clutch 302 isconsidered to be operating in its fully "actuated" condition. As such,drive torque is transferred from mainshaft 302 to drive sprocket 76through the interleaved clutch plates 108 and 110 to transmit drivetorque to front output shaft 88. Concurrent with such axial movement ofcrowned roller 132 along first cam pathway 358 toward its four-wheeldrive (4WD) position, range pin 418 is guided within first guideway 424of range slot 422 for maintaining shift sleeve 402 in the drive (D)position. Thus, first guideway 424 is a "dwell" slot having a commonradius centered on axis 126 for maintaining shift sleeve 402 in thedrive (D) position during axial movement of crowned roller 132 betweenits (2WD) and (4WD) positions. As discussed, when transfer case 400 isoperating in the on-demand mode, actuator 116 is actuated in accordancewith specific predefined relationships established in response to thecurrent value of the sensor input signals for rotatably driving sectorplate 412 between the 2WD and 4WD sector positions such that the amountof drive torque transferred through transfer clutch 302 is proportionalto the clutch engagement force, the value which is determined by theparticular position of crowned roller 132 between its (2WD) and (4WD)positions along first cam pathway 358.

According to the embodiment disclosed, power transfer system 10 isfurther operable to permit transfer case 400 to be shifted into thenon-driven or "Neutral" mode. More particularly, upon mode select means44 signalling selection of the "Neutral" mode, actuator 116 rotatessector plate 412 in the second direction (i.e., counterclockwise) untilcrowned roller 132 is guided along second cam pathway 428 of contourededge 340 while range pin 418 is concurrently guided within secondguideway 426 of range slot 422. Preferably, the contour of the secondcam pathway 428, which begins at point 356 and ends at point 430, isdesigned to retain crowned roller 132 in the two-wheel drive (2WD)position, whereby transfer clutch 302 is maintained in the non-actuatedcondition. More preferably, the contour of second cam pathway 428 is anarc having a common radius with its origin located on axis 126. Duringsuch rotation of sector plate 412, however, range pin 418 is axiallydisplaced due to the contour of second guideway 426 for axially movingshift sleeve 402 from the drive (D) position to the Neutral (N)position. Thus, during such axial movement of shift sleeve 402, drivemechanism 410 is adapted to maintain transfer clutch 302 in itsnon-actuated condition to eliminate the possibility of overloadingtransfer clutch 302. As will be appreciated, when mode select means 44signals that the vehicle operator wants to shift out of the Neutral modeand into one of the available drive modes, actuator 116 rotates sectorplate 412 in the first direction (i.e., clockwise) at least to the 2WDsector position wherein crowned roller 132 engages first cam pathway 358and range pin 418 is retained within first guideway 424.

In association with power transfer systems utilizing mode select means44, the present invention also incorporates means for maintaining theselected mode upon power interruption to actuator 116. To this end, abrake 230 is provided that is an electrically-controlled device operablein a "power-off" condition for braking output member 118 of actuator116. In operation, control module 150 delivers an electrical signal tobrake 230 to maintain it in a released or "power-on" condition. Duringcontrolled movement of output member 118, brake 230 is maintained in itsreleased "power-on" condition. However, upon interruption of power tobrake 230, brake torque is generated for inhibiting linear movement ofoutput member 118. Thus, once output member 118 is positioned in one ofits defined positions, power to brake 230 is interrupted for positivelyretaining sector plate 120 in the desired rotated position. Thereafter,power to rotary actuator 116 can be interrupted to minimize its on-timeservice requirements.

The foregoing discussion discloses and describes exemplary embodimentsof the present invention. One skilled in the art will readily recognizefrom such discussion, and from the accompanying drawings and claims,that various changes, modifications and variations can be made thereinwithout departing from the true spirit and fair scope of the inventionas defined in the following claims.

What is claimed is:
 1. A power transfer system for a motor vehiclehaving an engine, a first driveline interconnected to a first set ofwheels, and a second driveline interconnected to a second set of wheels,said power transfer system comprising:a transfer case having an inputshaft rotatably driven by the engine, a first output shaftinterconnected to the first driveline, a second output shaftinterconnected to the second driveline, a shift mechanism movablebetween an engaged position for coupling said first output shaft to saidinput shaft for establishing a drive mode and a released position foruncoupling said first output shaft from said input shaft forestablishing a non-driven mode, a transfer clutch for selectivelytransmitting drive torque from said first output shaft to said secondoutput shaft, said transfer clutch being operable for varying the drivetorque transmitted therethrough in relation to a corresponding variationin a clutch engagement force, a drive mechanism for generating saidclutch engagement force and which is movable between a first positionwhereat a minimum clutch engagement force is generated and a secondposition whereat a maximum clutch engagement force is generated, amovement coordinating apparatus for coordinating movement of said shiftmechanism and said drive mechanism, and an actuator for selectivelymoving said movement coordinating apparatus between a two-wheel driveposition whereat said shift mechanism is in said engaged position andsaid drive mechanism is in said first position, a four-wheel driveposition whereat said shift mechanism is in said engaged position andsaid drive mechanism is in said second position, and a neutral positionwhereat said shift mechanism is in said released position and said drivemechanism is in one of said first and second positions; sensor means forsensing the rotational speed of said first and second output shafts andrespectively generating first and second speed signals indicativethereof; mode select means for enabling a vehicle operator to select oneof an On-Demand drive mode and a Neutral mode and generate a mode signalindicative of the particular mode selected; and controller means forreceiving said first and second signals and generating a speeddifferential signal that is indicative of a speed differential betweensaid first and second output shafts, said controller means operable forcontrolling actuation of said actuator in accordance with said speeddifferential signal and said mode signal, said controller means causingsaid actuator to modulate the position of said movement coordinatingapparatus between said two-wheel drive and four-wheel drive positions asa function of the magnitude of said speed differential signal forvarying the torque transmitted through said transfer clutch when saidOn-Demand drive mode is selected, said controller means further operablefor causing said actuator to move said movement coordinating apparatusto said neutral position when said Neutral mode is selected.
 2. Thepower transfer system of claim 1 wherein said mode select means isoperable for enabling a vehicle operator to select a Part-TimeFour-Wheel drive mode in addition to said On-Demand drive mode and saidNeutral mode, whereby when said mode signals indicates that saidPart-Time Four-Wheel drive mode has been selected said controller meanscauses said actuator to move said movement coordinating apparatus tosaid four-wheel drive position.
 3. The power transfer system of claim 2wherein said mode select means is further operable to permit selectionof a Two-Wheel drive mode, whereby when said mode signal indicates thatsaid Two-Wheel drive mode has been selected said controller means causessaid actuator to move said movement coordinating apparatus to saidtwo-wheel drive position.
 4. A power transfer system for a motor vehiclehaving an engine, a first driveline interconnected to a first set ofwheels, and a second driveline interconnected to a second set of wheels,said power transfer system comprising:a transfer case having an inputshaft rotatably driven by the engine, a first output shaftinterconnected to the first driveline, a second output shaftinterconnected to the second driveline, a shift mechanism movablebetween an engaged position for coupling said first output shaft to saidinput shaft for establishing a drive mode and a released position foruncoupling said first output shaft from said input shaft forestablishing a non-driven mode, a transfer clutch for selectivelytransmitting drive torque from said first output shaft to said secondoutput shaft, said transfer clutch being operable for varying the drivetorque transmitted therethrough in relation to a corresponding variationin a clutch engagement force, a drive mechanism for generating saidclutch engagement force and which is operable in a first position forgenerating a minimum clutch engagement force and in a second positionfor generating a maximum clutch engagement force, a movementcoordinating apparatus for coordinating movement of said shift mechanismand said drive mechanism, and an actuator for selectively moving saidmovement coordinating apparatus between a two-wheel drive positionwhereat said shift mechanism is in said engaged position and said drivemechanism is in said first position, a four-wheel drive position whereatsaid shift mechanism is in said engaged position and said drivemechanism is in said second position, and a neutral position whereatsaid shift mechanism is in said released position and said drivemechanism is in said first position; sensor means for sensing therotational speed of said first and second output shafts and respectivelygenerating first and second speed signals indicative thereof; modeselect means for enabling a vehicle operator to select one of aPart-Time Four-Wheel drive mode, an On-Demand drive mode and a Neutralmode and generating a mode signal indicative of the particular modeselected; and controller means for receiving said first and second speedsignals and generating a speed differential signal that is indicative ofthe speed difference between said first and second output shafts, saidcontroller means operable for controlling actuation of said actuator inaccordance with said speed differential signal and said mode signal, andwherein said controller means is operable for causing said actuator tomove said movement coordinating apparatus to said four-wheel driveposition when said mode signal indicates selection of said Part-timeFour-Wheel drive mode, said controller means causing said actuator tomodulate the position of said movement coordinating apparatus betweensaid two-wheel drive and four-wheel drive positions as a function of themagnitude of said speed differential signal for varying the torquetransmitted through said transfer clutch when said On-Demand drive modeis selected, and said controller means operable for causing saidactuator to move said movement coordinating apparatus to said neutralposition when said Neutral mode is selected.
 5. The power transfersystem of claim 4 wherein during operation in said On-Demand drive modesaid controller means causes said actuator to move said movementcoordinating apparatus to said two-wheel drive position when the valueof said speed differential signal is less than a predetermined minimumvalue and said controller means causes said actuator to move saidmovement coordinating apparatus to said four-wheel drive position whenthe value of said speed differential signal exceeds a predeterminedmaximum value.
 6. The power transfer system of claim 4 wherein said modeselect means is further operable to permit selection of a Two-Wheeldrive mode whereby said controller means causes said actuator to movesaid movement coordinating apparatus to said two-wheel drive positionwhen said mode signal indicates selection of said Two-Wheel drive mode.7. A power transfer system for a motor vehicle having an engine, a firstdriveline interconnecting a first set of wheels and a second drivelineinterconnecting a second set of wheels, said power transfer systemcomprising:a transfer case having an input shaft rotatably driven by theengine, a first output shaft interconnected to the first driveline, asecond output shaft interconnected to the second driveline, a shiftmechanism movable between an engaged position for coupling said firstoutput shaft to said input shaft for establishing a drive mode and areleased position for uncoupling said first output shaft from said inputshaft for establishing a non-driven mode, a transfer clutch forselectively transmitting drive torque from said first output shaft tosaid second output shaft, said transfer clutch being operable forvarying the drive torque transmitted therethrough in relation to acorresponding variation in a clutch engagement force, a drive mechanismfor generating said clutch engagement force and which is movable betweena first position whereat a minimum clutch engagement force is generatedand a second position whereat a maximum clutch engagement force isgenerated, a movement coordinating apparatus for coordinating movementof said shift mechanism and said drive mechanism, and an actuator forselectively moving said movement coordinating apparatus between atwo-wheel drive position whereat said shift mechanism is in said engagedposition and said drive mechanism is in said first position, afour-wheel drive position whereat said shift mechanism is in saidengaged position and said drive mechanism is in said second position,and a neutral position whereat said shift mechanism is in said releasedposition and said drive mechanism is in said first position; sensormeans for sensing the rotational speed of the first and second outputshafts and respectively generating first and second speed signalsindicative thereof; mode select means for enabling a vehicle operator toselect one of a Two-Wheel drive mode, a Part-Time Four-Wheel drive mode,an On-Demand drive mode and a Neutral mode and generate a mode signalindicative of the particular mode selected; and controller means forreceiving said first and second speed signals and generating a speeddifferential signal that is indicative of a speed difference betweensaid first and second output shafts, said controller means operable forcontrolling actuation of said actuator in accordance with said speeddifferential signal and said mode signal, said controller means causingsaid actuator to move said movement coordinating apparatus to saidtwo-wheel drive position when said mode signal indicates selection ofsaid Two-Wheel drive mode, said controller means causing said actuatorto move said movement coordinating apparatus to said four-wheel driveposition when said mode signal indicates selection of said Part-TimeFour-Wheel drive mode, said controller means causing said actuator tomodulate the position of said movement coordinating apparatus betweensaid two-wheel drive and four-wheel drive positions as a function ofsaid speed differential signal for varying the torque transmittedthrough said transfer clutch when said On-Demand drive mode is selected,and said controller means operable for causing said actuator to movesaid movement coordinating apparatus to said neutral position when saidNeutral mode is selected.
 8. The power transfer system of claim 7wherein during operation in said On-Demand drive mode said controllermeans causes said actuator to move said movement coordinating apparatusto said two-wheel drive position when the value of said speeddifferential signal is less than a predetermined minimum value and saidcontroller means causes said actuator to move said movement coordinatingapparatus to said four-wheel drive position when the value of said speeddifferential signal exceeds a predetermined maximum value, and whereinsaid controller means is operable for increasing the magnitude of drivetorque transmitted through said transfer clutch in response toincreasing values of said speed differential signal between saidpredetermined minimum value and said predetermined maximum value.
 9. Apower transfer system for a motor vehicle having an engine, a firstdriveline interconnected to a first set of wheels, and a seconddriveline interconnected to a second set of wheels, said power transfersystem comprising:a transfer case having an input shaft rotatably drivenby the engine, a first output shaft interconnected to the firstdriveline, a second output shaft interconnected to the second driveline,a shift mechanism movable between an engaged position for coupling saidfirst output-shaft to said input shaft for establishing a drive mode anda released position for uncoupling said first output shaft from saidinput shaft for establishing a non-driven mode, a transfer clutch forselectively transmitting drive torque from said first output shaft tosaid second output shaft, said transfer clutch being operable forvarying the drive torque transmitted therethrough in relation to acorresponding variation in a clutch engagement force, a drive mechanismfor generating said clutch engagement force and which is operable in afirst position for generating a minimum clutch engagement force and in asecond position for generating a maximum clutch engagement force, amovement coordinating apparatus for coordinating movement of said shiftmechanism and said drive mechanism, and an actuator for selectivelymoving said movement coordinating apparatus between a two-wheel driveposition whereat said shift mechanism is in said engaged position andsaid drive mechanism is in said first position, a four-wheel driveposition whereat said shift mechanism is in said engaged position andsaid drive mechanism is in said second position, and a neutral positionwhereat said shift mechanism is in said released position and said drivemechanism is in said first position; sensor means for sensing therotational speed of said first and second output shafts and respectivelygenerating first and second speed signals indicative thereof; modeselect means for enabling a vehicle operator to select one of aTwo-Wheel drive mode, an On-Demand drive mode and a Neutral mode andgenerating a mode signal indicative of the particular mode selected; andcontroller means for receiving said first and second speed signals andgenerating a speed differential signal that is indicative of the speeddifference between said first and second output shafts controllingactuation of said actuator in accordance with said speed differentialsignal and said mode signal, and wherein said controller means isoperable for causing said actuator to move said movement coordinatingapparatus to said two-wheel drive position when said mode signalindicates selection of said Two-Wheel drive mode, said controller meanscausing said actuator to modulate the position of said movementcoordinating apparatus between said two-wheel drive and four-wheel drivepositions as a function of the magnitude of said speed differentialsignal for varying the torque transmitted through said transfer clutchwhen said On-Demand drive mode is selected, and said controller meansoperable for causing said actuator to move said movement coordinatingapparatus to said neutral position when said Neutral mode is selected.10. The power transfer system of claim 9 wherein during operation insaid On-Demand drive mode said controller means causes said actuator tomove said movement coordinating apparatus to said two-wheel driveposition when the value of said speed differential signal is less than apredetermined minimum value and said controller means causes saidactuator to move said movement coordinating apparatus to said four-wheeldrive position when the value of said speed differential signal exceedsa predetermined maximum value.
 11. The power transfer system of claim 9wherein said mode select means is further operable to permit selectionof a Part-Time Four-Wheel drive mode whereby said controller meanscauses said actuator to move said movement coordinating apparatus tosaid four-wheel drive position when said mode signal indicates selectionof said Part-Time Four-Wheel drive mode.
 12. A power transfer system fora motor vehicle having an engine, a first driveline interconnected to afirst set of wheels, and a second driveline interconnected to a secondset of wheels, said power transfer system comprising:a transfer casehaving an input shaft rotatably driven by the engine, a first outputshaft operably interconnected to the first driveline, a second outputshaft operably interconnected to the second driveline, a shift mechanismmovable between an engaged position for coupling said first output shaftto said input shaft for establishing a drive mode and a releasedposition for uncoupling said first output shaft from said input shaftfor establishing a non-driven mode, a transfer clutch for selectivelytransmitting drive torque from said first output shaft to said secondoutput shaft, said transfer clutch being operable for varying the drivetorque transmitted therethrough in relation to a corresponding variationin a clutch engagement force, a drive mechanism for generating saidclutch engagement force, said drive mechanism movable between a firstposition whereat a minimum clutch engagement force is generated and asecond position whereat a maximum clutch engagement force is generated,a movement coordinating apparatus for coordinating movement of saidshift mechanism and said drive mechanism, and an actuator forselectively moving said movement coordinating apparatus between atwo-wheel drive position whereat said shift mechanism is in said engagedposition and said drive mechanism is in said first position, afour-wheel drive position whereat said shift mechanism is in saidengaged position and said drive mechanism is in said second position,and a neutral position whereat said shift mechanism is in said releasedposition and said drive mechanism is in said first position; sensormeans for sensing the rotational speed of said first and second outputshafts and respectively generating first and second speed signalsindicative thereof; mode select means for enabling a vehicle operator toselect one of a Two-Wheel drive mode, a Part-Time Four-Wheel drive modeand a Neutral mode and generate a mode signal indicative of theparticular mode selected; and controller means for receiving said firstand second speed signals and generating a speed differential signal thatis indicative of a speed differential between the first and second setof wheels, said controller means operable for controlling actuation ofsaid actuator in accordance with said speed differential signal and saidmode signal, said controller means causing said actuator to move saidmovement coordinating apparatus to said two-wheel drive position whensaid mode signal indicates selection of said Two-Wheel drive mode, saidcontroller means causing said actuator to move said movementcoordinating apparatus to said four-wheel drive position when said modesignal indicates selection of said Part-Time Four-Wheel drive mode, andsaid controller means operable for causing said actuator to move saidmovement coordinating apparatus to said neutral position when saidNeutral mode is selected.
 13. The power transfer system of claim 12wherein said drive mechanism includes a lever arm supported forpivotable movement between said first and second position, and whereinsaid movement coordinating apparatus comprises a rotatable sector plateinterconnected to said lever arm such that rotation of said sector platetoward said two-wheel drive position causes corresponding pivotablemovement of said lever arm toward said first position and rotation ofsaid sector plate toward said four-wheel drive position causescorresponding pivotable movement of said lever arm toward said secondposition, and wherein said actuator is an electrically-controlled devicehaving a rotatable output member coupled to said sector plate forcontrolling rotation thereof in response to an electrical controlsignal.
 14. The power transfer system of claim 13 wherein said shiftmechanism includes a shift sleeve retained for rotation with and axialsliding movement on said first output shaft, and a shift assemblyinterconnecting said shift sleeve to said sector plate, whereby rotationof said sector plate from said neutral position into one of saidtwo-wheel drive and four-wheel drive positions cause correspondingmovement of said shift sleeve to said engaged position and rotation ofsaid sector plate to said neutral position causes corresponding movementof said shift sleeve to said released position.
 15. A power transfersystem for a motor vehicle having an engine, comprising:a firstdriveline having a first set of wheels; a second driveline having asecond set of wheels; a transfer case having an input shaft rotatablydriven by the engine, a first output shaft interconnected to said firstdriveline, a second output shaft interconnected to said seconddriveline, a shift sleeve movable between an engaged position whereatsaid first output shaft is coupled to said input shaft for establishinga drive mode and a released position whereat said first output shaft isuncoupled from said input shaft for establishing a non-driven mode, atransfer clutch for selectively transmitting drive torque from saidfirst output shaft to said second output shaft, said transfer clutchbeing operable for varying the drive torque transmitted therethrough inrelation to a corresponding variation in a clutch engagement force, adrive mechanism for generating said clutch engagement force, said drivemechanism movable between a first position whereat a minimum clutchengagement force is generated and a second position whereat a maximumclutch engagement force is generated, means for moving said shift sleeveand said drive mechanism including an actuator selectively movablebetween a two-wheel drive position whereat said shift mechanism is insaid engaged position and said drive mechanism is in said firstposition, a four-wheel drive position whereat said shift mechanism is insaid engaged position and said drive mechanism is in said secondposition, and a neutral position whereat said shift mechanism is in saidreleased position and said drive mechanism is in one of said first andsecond positions; sensor means for sensing the rotational speed of saidfirst and second output shafts and respectively generating first andsecond speed signals indicative thereof; mode select means for enablinga vehicle operator to select one of an On-Demand drive mode and aNeutral mode and generate a mode signal indicative of the particularmode selected; and controller means for receiving said first and secondspeed signals and generating a speed differential signal that isindicative of a speed difference between said first and second outputshafts, said controller means operable for controlling actuation of saidactuator in accordance with said speed differential signal and said modesignal, said controller means causing said actuator to modulate betweensaid two-wheel drive and four-wheel drive positions as a function of themagnitude of said speed differential signal for varying the torquetransmitted through said transfer clutch when said On-Demand drive modeis selected, said controller means further operable for moving saidactuator to said neutral position when said Neutral mode is selected.16. The power transfer system of claim 11 wherein said moving meansincludes a movement coordinating apparatus for coordinating movement ofsaid shift sleeve and said drive mechanism in response to movement ofsaid actuator.
 17. The power transfer system of claim 15 wherein duringoperation in said On-Demand drive mode said controller means causes saidactuator to move to said two-wheel drive position when the value of saidspeed differential signal is less than a predetermined minimum value andsaid controller means causes said actuator to move to said four-wheeldrive position when said speed differential signal exceeds apredetermined maximum value, and wherein said controller means isoperable for increasing the magnitude of drive torque transmittedthrough said transfer clutch in response to increasing values of saidspeed differential signal between said predetermined minimum value andsaid predetermined maximum value.
 18. The power transfer system of claim16 wherein said drive mechanism includes a lever arm supported forpivotable movement between said first and second position, and whereinsaid movement coordinating apparatus comprises a rotatable sector plateinterconnected to said lever arm such that rotation of said sector platetoward said two-wheel drive position causes corresponding pivotablemovement of said lever arm toward said first position and rotation ofsaid sector plate toward said four-wheel drive position causescorresponding pivotable movement of said lever arm toward said secondposition, and wherein said actuator is an electrically-controlled devicehaving a rotatable output member coupled to said sector plate forcontrolling rotation thereof in response to an electrical control signalthe magnitude of which is a function of the magnitude of said speeddifferential signal.
 19. The power transfer system of claim 18 whereinsaid shift sleeve is retained for rotation with and axial slidingmovement on said first output shaft and is operably interconnected tosaid sector plate, whereby rotation of said sector plate from saidneutral position into one of said two-wheel drive and four-wheel drivepositions cause corresponding movement of said shift sleeve to saidengaged position and rotation of said sector plate to said neutralposition causes corresponding movement of said shift sleeve to saidreleased position.
 20. A power transfer system for a motor vehiclecomprising:a first driveline having a first set of wheels; a seconddriveline having a second set of wheels; a transfer case having an inputshaft rotatably driven by the engine, a first output shaftinterconnected to said first driveline, a second output shaftinterconnected to said second driveline, a shift sleeve movable betweenan engaged position whereat said first output shaft is coupled to saidinput shaft for establishing a drive mode and a released positionwhereat said first output shaft is uncoupled from said input shaft forestablishing a non-driven mode, a transfer clutch for selectivelytransmitting drive torque from said first output shaft to said secondoutput shaft, said transfer clutch being operable for varying the drivetorque transmitted therethrough in relation to a corresponding variationin a clutch engagement force, a drive mechanism for generating saidclutch engagement force, said drive mechanism movable between a firstposition whereat a minimum clutch engagement force is generated and asecond position whereat a maximum clutch engagement force is generated,means for moving said shift sleeve and said drive mechanism including anactuator selectively movable between a two-wheel drive position whereatsaid shift mechanism is in said engaged position and said drivemechanism is in said first position, a four-wheel drive position whereatsaid shift mechanism is in said engaged position and said drivemechanism is in said second position, and a neutral position whereatsaid shift mechanism is in said released position and said drivemechanism is in one of said first and second positions; sensor means forsensing the rotational speed of said first and second output shafts andrespectively generating first and second speed signals indicativethereof; mode select means for enabling a vehicle operator to select oneof a Four-Wheel drive mode, an On-Demand drive mode and a Neutral modeand generate a mode signal indicative of the particular mode selected;and controller means for receiving said first and second speed signalsand generating a speed differential signal that is indicative of a speeddifference between said first and second output shafts, said controllermeans operable for controlling actuation of said actuator in accordancewith said speed differential signal and said mode signal, saidcontroller means causing said actuator to move to said four-wheel driveposition when said mode signal indicates selection of said Four-Wheeldrive mode, said controller means causing said actuator to modulatebetween said two-wheel drive and four-wheel drive positions as afunction of the magnitude of said speed differential signal for varyingthe torque transmitted through said transfer clutch when said On-Demanddrive mode is selected, said controller means further operable formoving said actuator to said neutral position when said Neutral mode isselected.
 21. The power transfer system of claim 20 wherein said movingmeans includes a movement coordinating apparatus for coordinatingmovement of said shift sleeve and said drive mechanism in response tomovement of said actuator.